1. Field of the Invention
The present invention relates to a grinding apparatus comprising a vertical parting vessel composed of an inner rotor part and an outer part forming a stator or another rotor rotating in a reverse direction relative to the inner rotor. The parting vessel generates a special centrifugal fluidization of steel balls used therein as the grinding medium to grind starting materials therein.
2. Description of the Related Art
Of the known grinding apparatuses, there are the tube mill type, vertical mill type, or the like. Among these conventional grinding apparatuses, is a vertical ball mill having a rotor forming an integrated vessel with a cover, in which rotor a grinding medium such as steel balls (hereinafter referred to as "balls") are circulated to grind starting materials by compression and/or impact and shearing in cooperation with the covered vessel.
Japanese Patent Unexamined Publications (KOKAI) Nos. 57-209649 and 58-3650 disclose vertical ball mills of the continuation type. For convenience of explanation herein, FIGS. 2a and FIG. 2b attached hereto show basic and general profiles of the disclosed ball mills, respectively.
FIG. 2a is a cross-sectional view of the vertical ball mill taken along a vertical plane including the vertical axis X of the mill. Numeral 1 denotes a rotational vessel symmetrical about the vertical axis and having a horizontal bottom surface B and a circumferential sloping side surface A, the diameter of which increases as the center of the diameter moves upwards along the vertical axis X. The vessel 1 is closed by a cover 3. The cover 3 has a circumferential concave portion in a ring form and a central planner portion integrated therewith. The concave portion opens downwardly and has a semi-circular profile in a vertical cross-sectional view. The vessel 1 is provided with a drive shaft 2 for rotating the vessel.
With this arrangement, the balls are forced to creep up the sloping surface A, due to rotation of the vessel 1, from the bottom surface, move along the inner surface of the concave ring portion, and then fall and strike the bottom surface B. This circulation of the balls is effected repeatedly in a cross-sectional view taken along any vertical plane including the vertical axis, while the vessel 1 rotates.
FIG. 2b is a cross-sectional view corresponding to FIG. 2a and showing another conventional vertical ball mill. This mill is substantially the same as that of FIG. 2a except for a configuration of a vessel 4. The vessel 4 has a circumferential groove portion and a central solid portion 5 of a truncated cone projecting upward. The inner circumferential surface A of the groove portion and the outer circumferential surface C of the central solid portion 5 form reversed cones projecting downwards in a cross-sectional view taken along a vertical plane including the axis X. The groove portion has a circumferential bottom surface B. According to the other ball mill, the balls fall from the concaved inner surface of the cover 3 to strike the sloping surface C and then bounce toward the bottom surface B.
According to the conventional ball mills as shown in FIG. 2a and FIG. 2b, grinding is effected mainly by the action of the balls sliding against the surface A of the rotational vessel 1. The sliding movement is divided into two components: one is an upward slide of the balls; and the other one is a slide of the balls caused by a difference between a velocity of the surface in a tangential direction and a velocity of the balls revolving about the vertical axis in a tangential direction.
In this connection, the surface A of the conventional vertical ball mill rotates in the same tangential direction as that in which the balls revolve, since the surface A forms an integral part of the rotary vessel 4. Under the circumstances, the difference in the tangential velocity between the surface A and the balls is not large, and as a result, the effect of the grinding by compression and/or impact and shearing due to the tangential velocity difference is low.
In addition, the rotations of the vessels 1 and 4 generate centrifugal forces which are imparted to the balls, and the balls are forced to creep up along the surface A due to the centrifugal force and store potential energy. With the ball mill of FIG. 2a, the potential energy, however, is almost lost or consumed when the balls fall to the bottom surface B from the ceiling surface of the cover 3, with the result that the balls have little energy to effect grinding at the surface B.
With the ball mill, as shown in FIG. 2b, the balls fall onto the sloping surface C from the ceiling surface of the cover 3 and rebound from the surface C, thus obtaining a radial force. This means that some of the potential energy of each ball is transformed to kinetic energy, which is utilized for the grinding by compression and/or impact and shearing. However, a great deal of the potential energy is lost when the ball strikes the sloping surface C.
As explained above, the conventional grinding apparatus, i.e., the vertical ball mills, have a problem in that the energy given to the grinding apparatus is not effectively utilized for the grinding by compression and/or impact and shearing, but is consumed to a great extent as heat energy, that is, the energy effect for grinding is low.